Inverter

ABSTRACT

An inverter ( 1 ) for feeding electric power into a utility grid ( 7 ) or into a load is described. The inverter ( 1 ) contains two direct voltage inputs ( 2, 3 ), one first intermediate circuit ( 8 ) connected thereto and comprising two series connected capacitors (C 1,  C 2 ) that are connected together at a ground terminal ( 14 ), two alternating voltage outputs ( 5, 6 ) of which one at least is provided with a grid choke (L 1 ) and one bridge section ( 10 ). In accordance with the invention, a second intermediate circuit ( 9 ) that is devised at least for selectively boosting the direct voltage and is intended for supplying the bridge section ( 10 ) with positive and negative voltage is interposed between the first intermediate circuit ( 8 ) and the bridge section ( 10 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims Priority from German Application No. DE 10 2007038 959.2-32 filed on 14 Aug. 2007

FIELD OF THE INVENTION

The invention relates to an inverter of the type mentioned in thepreamble of claim 1. voltage generators such as photovoltaic or fuelcell plants into an alternating current grid, in particular into theutility grid (50/60 Hz), inverters of various types are used. In mostcases, a direct voltage converter (DC-DC controller) is provided betweenthe direct voltage generator and the inverter and serves the purpose ofconverting the direct voltage delivered by the direct voltage generatorinto a direct voltage needed by the inverter or adapted thereto.

For different reasons, it is desired to ground one of the outputs of thedirect voltage generator or to fix the potential of the direct voltagegenerator with respect to ground potential. The reason therefor is onthe one side that such grounding is prescribed in some countries. On theother side, in the absence of grounding, diverse disadvantages occurduring operation. One of them is the problem of high frequency leakagecurrents. Unavoidable, parasitic capacitances between the direct voltagegenerator and ground may give rise to considerable equalizing currentsin case of potential fluctuations, said equalizing currents constitutingan intolerable safety risk which make it necessary to provide forcomplex monitoring provisions with the help of error current sensors orthe like for contact protection or for achieving electromagneticcompatibility (EMC) and which can only be securely avoided throughgrounding. Potential fluctuations at the direct voltage generator canfurther lead to permanent destruction of certain solar modules such asthin film modules or the like.

In principle, leakage currents of the type described can be readilyavoided if direct voltage converters with transformers are being used togalvanically separate the direct voltage side from the alternatingvoltage side. However, independent on whether grid transformers or highfrequency transformers are being used, transformers result in areduction of efficiency, in parts considerable weight and size and/or anadditional control expense, this being the reason why transformerlessvoltage converters are being preferred. The usual topologies oftransformerless voltage converters however make the desired groundingeither impossible since it would cause needed switches, capacitances orthe like to short-circuit or result in an increased switching expenseand in other drawbacks.

DESCRIPTION OF THE PRIOR ART

Therefore, numerous tests have already been conducted in order to avoidin another way the occurrence of the disadvantages mentioned. Circuitsare known in particular, which serve the purpose of reducing theundesired leakage currents (e.g., DE 10 2004 037 466 A1, DE 102 21 592A1, DE 10 2004 030 912 B3) . In these circuits, a solar generator ise.g., operated in certain phases of the inner electric power transport,isolated from the grid. As the solar generator is periodicallyelectrically connected to the grid, the charge of its parasiticcapacitances is only slightly reversed so that the potential of thesolar generator changes sinusoidally at grid frequency and at a voltageamplitude that corresponds to half the grid voltage. High frequencycurrents then only form through the slight voltage differences of thesolar generator between two switching cycles and through asymmetriesduring switching. Capacitive leakage currents can thus be stronglyminimized but cannot be completely avoided.

Further, a circuit arrangement is known (DE 102 25 020 A1), which uses adivided solar generator the center point of which is grounded. As aresult, all the parts of the generator have a fixed potential andcapacitive leakage currents cannot flow in principle. Since the twodirect current sources have different outputs, a circuit forcompensating the output differences and the voltages is moreoverprovided. In this proposed circuit, the disadvantages encountered arethe high voltage differences in the solar generator and at the switches,the additional losses in the compensation circuit and the fact that atleast four switches timed at high frequency are needed.

Besides, circuit arrays are already known by means of which a solargenerator can be grounded on one side, despite the absence of atransformer. As a result, capacitive leakage currents are prevented as amatter of principle. One of these circuit arrays (DE 196 42 522 C1)however needs five active switches, one or two switches having to switchsimultaneously at high frequency and to provide the mean output current.On this circuit, which is also referred to as a “flying inductor”, theefficiency is negatively affected by the high number of componentssimultaneously participating in series in the current flow. Thedisadvantage of this circuit is that intermittent current pulses areimpressed upon the grid, said pulses requiring a capacitive grid filterwhich, as a matter of principle, degrades not only the power factor butalso the efficiency of the circuit in the part load range due to its ownfreewheeling power need. Although such a capacitive grid filter can beavoided with another known circuit (DE 197 32 218 C1), nine activeswitches are needed for this purpose of which two at least must beswitched simultaneously at high frequencies so that the expense in termsof construction is even further increased and both the robustness andthe efficiency of the overall device is negatively affected. Thetopology of a flying inductor further has the disadvantage that thevoltage load of the switches depends on the grid voltage and issensitive to grid failures.

Finally, devices are known (US 2007/0047277 A1) that are configured withtwo stages and that comprise, beside the actual inverter (DC-ACconverter), a direct voltage or a DC-DC converter. The inverters areprovided with a bipolar voltage intermediate circuit containing twoseries-connected capacitors that are connected together at a groundterminal associated with the neutral conductor of the respective gridand connected therewith. In this case, the ground terminal of theinverter can moreover be connected with the negative output of thedirect voltage generator. This is made possible using a special storagechoke composed of two magnetically coupled windings.

The advantage that this device allows for grounding the direct voltagegenerator with relatively simple means, in particular withouttransformer, is opposed by the disadvantage that it needs at least threeactive switches clocked at high frequency and that it is formed with twostages, which increases the expense in terms of controlling, results inunavoidable losses and impairs efficiency.

BRIEF SUMMARY OF THE INVENTION

In view of this prior art, the technical problem of the invention is todevise the inverter of the type mentioned above in such a manner thatthe potential of the direct voltage generator can be fixed with respectto ground potential not only with relatively simple means in terms ofconstruction but also with a small number of components and withrelatively small loads at least for the switches that must be switchedat high frequency.

The characterizing features of claim 1 serve to solve this problem.

The invention proposes an inverter in a one-stage construction, i.e., aninverter in which the DC-DC part and the DC-AC part are combined into acombined circuit array with the possibility of boosting and bucking theinput voltage. As a result, a common control is made possible in onesingle stage. Moreover, an inverter with only two high frequencyswitches is provided. Finally, the potential of the direct voltagegenerator can be fixed with respect to ground potential and power can befed into the grid with a non intermittent current. By virtue of therelatively small number of components one further achieves highreliability and long useful life of the inverter.

Further advantageous features of the invention will become apparent fromthe dependent claims.

The invention will be best understood from the following descriptionwhen read in conjunction with the accompanying drawings. In saiddrawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the circuit diagram of an inverter of the invention withswitches essential to its function; particular without transformer, isopposed by the disadvantage that it needs at least three active switchesclocked at high frequency and that it is formed with two stages, whichincreases the expense in terms of controlling, results in unavoidablelosses and impairs efficiency. BRIEF SUMMARY OF THE INVENTION

In view of this prior art, the technical problem of the invention is todevise the inverter of the type mentioned above in such a manner thatthe potential of the direct voltage generator can be fixed with respectto ground potential not only with relatively simple means in terms ofconstruction but also with a small number of components and withrelatively small loads at least for the switches that must be switchedat high frequency.

The characterizing features of claim 1 serve to solve this problem.

The invention proposes an inverter in a one-stage construction, i.e., aninverter in which the DC-DC part and the DC-AC part are combined into acombined circuit array with the possibility of boosting and bucking theinput voltage. As a result, a common control is made possible in onesingle stage. Moreover, an inverter with only two high frequencyswitches is provided. Finally, the potential of the direct voltagegenerator can be fixed with respect to ground potential and power can befed into the grid with a non intermittent current. By virtue of therelatively small number of components one further achieves highreliability and long useful life of the inverter.

Further advantageous features of the invention will become apparent fromthe dependent claims.

The invention will be best understood from the following descriptionwhen read in conjunction with the accompanying drawings. In saiddrawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the circuit diagram of an inverter of the invention withswitches essential to its function;

FIG. 2 through 4 show the operation of the inverter shown in FIG. 1 forpositive output voltage and positive output current with three differentswitching states of the switches;

FIG. 5 shows the curve of currents occurring at different places duringoperation of the inverter of FIG. 1;

FIG. 6 schematically shows an exemplary embodiment of the inverter ofthe invention, modified over FIG. 1; and

FIG. 7 shows an inverter of a construction analogous to that of FIG. 6but with three phases.

DETAILED DESCRIPTION OF THE INVENTION

According to FIG. 1, a one-phase inverter 1 of the invention containstwo inputs 2 (positive) and 3 (negative) intended to apply a directvoltage, said inputs being connected e.g., to the corresponding outputsof a direct voltage generator 4 in the form of a photovoltaic or fuelcell plant, a capacitor that has not been illustrated can be connectedin parallel to said outputs as usual.

The inverter 1 further contains two outputs 5 and 6 that serve fordelivering an alternating voltage and for connection e.g., to aschematically illustrated, one-phase utility grid 7 or to a load. Asmoothing or grid choke L1 can be connected upstream of at least one ofthe outputs, this applying in the exemplary embodiment for the output 5that is connected to phase L of the grid 7.

As contrasted with most of the known circuit arrays, no additional andseparate DC-DC converter is interposed between the direct voltagegenerator 4 and the inverter 1. Instead, an inverter is proposed inaccordance with the invention, which not only performs a DC-ACconversion as shown in FIG. 1, but is also suited for boosting orbucking the input voltage to a desired level, i.e., which also has theproperties of the DC-DC converter with boosting-bucking function. Bothfunctions are brought together in the inverter 1 of the invention. Forthis purpose, the inverter 1 comprises a first intermediate circuit 8, asecond intermediate circuit 9 and a bridge section 10.

The first intermediate circuit 8 is connected to the two inputs 2 and 3and contains two series-connected capacitors C1 and C2 that areconnected with their one terminals 11, 12 to a respective one of theinputs 2 and 3 and with their other terminal to each other. Thisconnecting point is at the same time a ground terminal 14 that has to bebrought to ground potential. The intermediate circuit 8 thus is anactually known, bipolar intermediate voltage circuit that is devised forthe bridge section 10 to be fed from a positive source C1 (topconnection 11, positive to ground) and from a negative source C2 (lowerconnection 12, negative to ground). By grounding to ground terminal 14and by using relatively high capacitances C1 and C2, one moreoverachieves that the potential of the direct voltage generator 4 isrelatively constant and that, even if there are parasitic capacitancesto ground, no substantial stray currents are obtained.

The second intermediate circuit 9 contains a first series memberconsisting of a first diode D3 connected to terminal 11 and of a firststorage choke L3 connected in series therewith, as well as a secondseries member consisting of a second diode D4 connected to terminal 12and of a second storage choke L2 connected in series therewith. Theother terminal of the storage choke L3 is connected to a first input 15,the other terminal of the storage choke L2 to a second input 16 of thebridge section 10. Moreover, the second intermediate circuit 9 containsa capacitor C4 that is connected on the one side with a connection point17 between the diode D3 and the storage choke L3 of the first seriesmember and on the other side with the input 16, as well as a capacitorC3 that is connected on the one side with a connecting point 18 betweenthe diode D4 and the storage choke L2 of the second series member and onthe other side with the input 15 of the bridge section 10.

The bridge section 10 is substantially formed by two series-connectedswitches S1 and S2, each comprising one first terminal connected withthe input 15 or 16 and one second terminal. The two second terminals areconnected together at a common connecting point 19 that is locatedbetween the two switches S1, S2 and that is connected to the one output5 of the inverter 1 through the grid choke L1 thus leading, through thegrid choke L1, to phase L of the grid 7. The normal conductor N of thegrid 7 is connected to the output 6 of the inverter 1 and from therethrough a line 20 to the connecting point 14 between the two capacitorsC1 and C2 and is brought to ground potential like this connecting point14.

In the exemplary embodiment, the second intermediate circuit 9 furthercomprises two freewheeling paths that are each formed by one additionalswitch S3 and S4 respectively and by one diode DS and D6 respectivelythat are connected in series therewith. The freewheeling path consistingof S3 and D5 lies between the connecting points 14 and 19, the diode D5being conductive in the direction of the connecting point 19. The secondfreewheeling path S4 and D6 also lies between the two connecting points14 and 19, but here, the diode D6 can only be made conductive in theopposite direction, meaning in the direction of connecting point 14.

Finally, one diode D1 and D2 can be connected in parallel with arespective one of the switches S1, S2 in the bridge section, the diodeD1 being conductive in the direction of the input 15 and the diode D2,in the opposite direction toward the connecting point 19 between the twoswitches S1, S2.

The switches S1, S2 are switched at high frequency, e.g., at a frequencyof 16 kHz or more, e.g., of 100 kHz, i.e., they are brought alternatelyinto a conductive and into a non-conductive state, whereas the twoswitches S3 and S4 are only switched on and off at the mains frequencyof e.g., 50 Hz or 60 Hz. In the exemplary embodiment, during thepositive half wave of the current to be fed into the grid 7, switch S3is in the conductive state and switch S4 in the non-conductive state,whilst during the negative half wave of the current to be fed into thegrid 7, the situation is reversed so that switch S4 is conductive andswitch S3, by contrast, non-conductive.

Generally, the basic idea of the invention is to provide twointermediate circuits 8 and 9 that are joined together. The firstintermediate circuit 8 is connected to the DC generator 4 on the inputside and substantially only consists of a storage configured to be acapacitive voltage splitter the connecting point 14 of which isconnected to ground potential. As a result, the potential of the DCgenerator 4 is fixed with respect to ground potential, preferablysymmetrically, with C1 being chosen to equal C2. By contrast, as will bediscussed in closer detail herein after, the second intermediate circuit9 serves on the one side to supply the bridge section 10 with thenecessary electrical voltages which are positive or negative withrespect to ground potential and on the other side e.g., to boost theoutput voltage of the first intermediate circuit 8 to the value desiredfor grid or load feeding by setting its output voltage by selecting theratio at which the two switches S1, S2 are switched on simultaneously.In the exemplary embodiment, the second intermediate circuit 9 moreoverhas the function of providing a freewheeling path for the currentflowing through the grid choke L1. All this is achieved by an integratedcircuit array that only needs a small number of switches, provides for asmall voltage load on the switches and allows for non-intermittent,continuous current feed into the grid 7. Moreover, the components D3,L3, C4, D5 on the one side and D4, L2, C3 and D6 on the other side areconfigured to be preferably completely symmetrical so that identicalconditions are achieved for the current flows during the positive andnegative half waves. Generally, an inverter 1 is thus obtained whichcomprises only two switches S1, S2 that are connected at high frequencyand are subjected to relative low load.

The functioning of the inverter 1 described will be discussed in closerdetail herein after with respect to the FIGS. 2 through 5 for the casein which one has a positive half wave, i.e., a positive output voltageis applied at the connecting point 17 and in which positive current isfed into the grid 7 through the grid choke L1.

We first assume that all the switches S1 through S4 are open. Then,after a short equalization, the operating condition becomes stationaryas soon as the two capacitors, if C1=C2, are charged to the halfgenerator voltage and the capacitors C3 and C4 are charged to fullgenerator voltage via the current paths D3, C4, L2, D4 and D3, L3, C3and D4 respectively. Now, no current flows through the storage chokes L2and L3 and in FIG. 4, the capacitor C4 has its positive side on the leftand the capacitor C3 on the right.

In order to allow for the boosting function, the two switches S1 and S2are switched on simultaneously (overlapping), as is shown in FIGS. 2 and5 for a time interval t3 and a phase A. Closing the switches S1, S2results in the components C4 and L3 on the one side and C3, L2 on theother side to be connected in parallel or short-circuited. As a result,the capacitor C4 is discharged via a current path from C4 via L3, S1, S2and back to C4 through the storage choke L3 and L3 is chargedaccordingly at the same time. Moreover, the capacitor C3 is dischargedvia a current path from C3 via S1, S2, L2 and back to C3 so that thestorage choke L2 is charged. As a result, the currents S1 and S2increase progressively in accordance with FIG. 5. Finally, a currentflowing through the grid choke L1 toward the grid 7 and having beengenerated in L1 in a previous phase can continue to flow via afreewheeling path L1, 7, 20, 14, S3, D5, 19 and back to L1 since theswitch L3 is permanently closed during the positive half wave. This canalso be seen from FIG. 5 since S3 carries a current during t3 and allowsfor an freewheeling current that must not flow via the uncoupling andcoupling diodes D3, D4. The freewheeling S3, D5 described would not benecessary if L1 were missing, which could be envisaged in principle.However, the grid choke L1 offers the advantage that it participates insmoothing the current to be delivered into the grid 7 and that itprevents this current from growing excessively. As further shown in FIG.5, phase A leads to a progressive increase of the currents flowingthrough the switches S1, S2. The power flow in the second intermediatecircuit 9 is prevented from being reversed by the locking action of thediodes D3 and D4.

In a following phase B, the switch S1 continues to be conductive duringa first part of the time interval t1 (FIGS. 3 and 5), whilst the switchS2 is being brought into the opened condition. As a result, theshort-circuit of L3, C4 and L2, C3 respectively is abolished so thatpower can now be transferred into the grid 7, starting from C1 via D3,L3, S1, 19, L1 to 7 and from there back via 20, 14 to C1. At the sametime, the storage chokes L2, L3 can deliver their stored energy to thecapacitors C3, C4, which are also charged by the capacitors C1, C2, thusgenerating at the connecting point 19 a higher voltage than the one atC1. With only two high frequency switches S1, S2, an inverter can thusbe constructed that can feed without transformer into the grid from avoltage source the output voltage of which is smaller or higher than thepeak value of the grid voltage.

According to FIG. 5, the current flowing through the grid choke L1 alsoflows through switch S1 whilst a decreasing current flows through D3 andD4, this being due to the progressive charge reversal L3 to C3 and L2 toC4. FIG. 5 moreover shows that the current through D4 is relativelysmall since, as contrasted with D3, the grid or load current carried byL1 does not flow through D4.

In the second part of the time interval t1 (phase C in FIG. 5) switch S1is closed and switch S2 is open during phase B so that power is suppliedto the grid 7 from the first intermediate circuit 8 via D3 and S1, L1.Like through S1, only the grid current still flows through the diode D3,said grid current being relatively constant and flowing from C1 via D3,L3, S1, 19, L1, 7, 20 and back to C1.

In a next step, which takes place during a time interval t* (see FIG. 4and phase D in FIG. 5), the switch S1 is open, whereas the switch S2remains open. As a result, the feeding cycle via S1, L1 is interrupted.The power stored in the grid choke L1 can however be dissipated via thefirst freewheeling path S3, D5, which is active now like in phase A andwhich allows for a current flow from L1 via 7, 20, S3, D5 and 19 back toL1. In FIG. 5, this manifests itself on the one side by a currentthrough S3 that is almost unchanged over the end of phase C. On theother side, progressively decreasing currents, which, in phase D, aredue to the fact that the capacitor C3 is further charged by the storagechoke L3, which was conductive in the previous phase and whichprogressively discharges as a result thereof, now flow through thediodes D3 and D4.

Phase D is followed by other cycles with the phases A through D as shownin FIG. 5.

When a negative current is fed into the grid during the negative halfwaves, the inverter 1 described functions substantially in the same way,although circumstances complementing FIGS. 2 through 5 are obtained.This more specifically means that switch S4 is closed and switch S3 openduring the negative half waves in order to provide an freewheeling pathfor a current flowing through the grid choke L1 in the oppositedirection. On the other side, switch S2 is now kept closed in the phasesB and C and switch S1 is opened instead. As a result, an electriccircuit is active during phase B that leads from C2 via D4, L2, S2, L1and 7 back to C2. In the phases A and B by contrast, an freewheelingpath is active which leads from L1 via S2, L2, D4, C2, 20 and 7 back toL1.

During the negative half waves also, the capacitors C3, C4 are boostedto a preselected value by the high switching frequencies and by apreselected duty cycle determining the boosting degree. Theoretically,the voltage C3 and C4 would continue to increase both during thepositive and during the negative half wave. Since power is permanentlydrawn from the capacitors C3, C4 by feeding power into the grid 7 orthrough a resistive load, the voltages at C3, C4 are prevented fromincreasing above a preselected value.

Irrespective thereof, the storage choke L3 is more strongly chargedduring the positive half waves and storage choke C2, during the negativehalf waves.

Generally, one thus obtains an inverter 1 capable of delivering anoutput voltage that is variable within wide limits without a usual DC-DCconverter and with only two high frequency switches S1, S2. The boostingfunction is thereby achieved by reversing the charge of the magneticstores L2, L3 and the capacitive stores C3, C4 and substantially by thefact that the power stored in the storage chokes L2, L3 when theswitches S1 and S2 are closed is delivered to the capacitors C1, C4 byopening at least one of the two switches S1, S2 in order to charge saidcapacitors to an intermediate circuit voltage that is higher than theinput voltage.

An exemplary embodiment of an inverter 21 is shown in FIG. 6 thatcontains, in analogous fashion as in FIG. 1, the first intermediatecircuit 8 connected to the direct voltage generator 4 and the bridgesection 10 intended for connection to the grid 7. Further, a secondintermediate circuit 22 is connected between the first intermediatecircuit 8 and the bridge section 10. As contrasted with FIG. 1, the twofreewheeling paths S3, D5 and S4, D6 are not integrated in the secondintermediate circuit 22 but are disposed separately and outside of thesecond intermediate circuit 22. For this purpose, a line 23 leads fromthe connecting point 14 between the two capacitors C1, C2 (FIG. 1) tothe input of a parallel circuit composed of S3, D5 and S4, D6. Thisparallel connection is e.g., interposed between the second intermediatecircuit 22 and the bridge section 10 and is connected at its output tothe connecting point 19 between the two switches S1, S2 (FIG. 1) of thebridge section 10. There is no difference in the function as describedherein above.

The control signals for the switches S1, S2 are generated appropriatelyduring the time span t3, t1 and t* shown in FIG. 5 using the usual meansused for PWM controls, the objective being to approximate as close aspossible the output current of the inverter 1 to a sinus shape. For thispurpose, a reference signal in the form of a triangular or a sawtoothsignal can be compared for example with a target value signal, thereference signal being generated separately for each polarity of theoutput voltage.

FIG. 7 schematically shows the structure of a three-phase inverter 24.This structure is obtained by the fact that for each phase of the grid 7a separate second intermediate circuit 22 a, 22 b and 22 c is provided,which could be provided with one external freewheeling group each.Alternatively however, three second intermediate circuits 9 configuredin accordance with FIG. 1 could also be used. Moreover, a bridge section10 a, 10 b and 10 c configured in accordance with the FIGS. 1 and 6 isconnected to each second intermediate circuit 22 a, 22 b and 22 c, saidbridge section supplying one of the three phases of the grid withcurrent.

The invention is not limited to the exemplary embodiments described,which can be varied in many ways. It is clear in particular that in thedescription given herein above only those components were described thatwere necessary to garner an understanding of the invention, and that inparticular the necessary and actually known controls, MPP controls andso on can be provided additionally. Moreover, it is understood that thevarious components can also be used in other combinations than thosedescribed and illustrated.

1. An inverter for feeding electric power into a utility grid (7) orinto a load, containing two inputs (2, 3) intended for applying a directvoltage, one first intermediate circuit (8) connected thereto andcomprising two series connected capacitors (C1, C2) that are connectedtogether at a ground terminal (14), two outputs (5, 6) intended fordelivering an alternating voltage of which one at least is provided witha grid choke (L1) and one bridge section (10) intended for convertingthe direct voltage into the alternating voltage, characterized in thatbetween the first intermediate circuit (8) and the bridge section (10)there is interposed a second intermediate circuit (9, 22) that isdevised at least for selectively boosting the direct voltage and isintended for supplying the bridge section (10) with positive andnegative voltage.
 2. The inverter as set forth in claim 1, characterizedin that the bridge section (10) is configured to be a half bridge andcontains two series connected switches (S1, S2) that are connected tofirst terminals (15, 16) parallel to the first intermediate circuit (8)and that lie at second terminals on one common connecting point (19),said connecting point (19) being connected to the one output (5) and theconnecting point (14) being connected to the other output (6) betweenthe two capacitors (C1, C2).
 3. The inverter as set forth in claim 1,characterized in that the second intermediate circuit (9) contains twofreewheeling paths (S3, D5; S4, D6) to maintain currents flowing throughthe grid choke (L1) in opposite directions.
 4. The inverter as set forthin claim 3, characterized in that the two freewheeling paths areconnected on the one side to the connecting point (14) of the twocapacitors (C1, C2) and on the other side to the connecting point (19)of the two switches (S1, S2) and contain each one switch (S3, S4) andone diode (D5, D6) connected in series therewith, said diodes (D5, D6)being conductive in opposite directions.
 5. The inverter as set forth inclaim 1, characterized in that the second intermediate circuit (9)contains one first and one second series member comprising one diode(D3, D4) and one storage choke (L3, L2) connected in series therewith,said first series member (D3, L3) leading from one input (11) to thefirst terminal (15) of one of the two switches (S1) and via one thirdcapacitor (C3) to a connecting point (18) between the storage choke (L2)and the diode (D4) of the second series member, whilst the second seriesmember (D4, L2) leads from the other input (12) to the first terminal(16) of the other one of the two switches (S2) and via one fourthcapacitor (C4) to a connecting point (17) between the storage choke (L3)and the diode (D4) of the first series member.
 6. The inverter as setforth in claim 5, characterized in that the two series members (D3, L3;D4, L2) on the one side and the associated capacitors (C3, C4) on theother side are built identically.
 7. The inverter as set forth in claim1, characterized in that the switches (S1, S2) of the bridge section(10) are switched at high frequency and the switches (S3, S4) of thefreewheeling paths, at grid frequency.
 8. The inverter as set forth in3, characterized in that the freewheeling paths (S3, D5; S4, D6) aredisposed outside of the second intermediate circuit (22).
 9. Theinverter as set forth in claim 1, characterized in that it is devisedfor connection to a three-phase grid and comprises three parallel secondintermediate circuits (22 a, 22 b, 22 c) associated with a respectivephase of the grid and connected to the first intermediate circuit (8).